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By Jatindra Dash

BHUBANESHWAR, India (Reuters) - Indian police killed at least 18 Maoist rebels in the eastern state of Odisha on Monday after tracking them to a remote, forested area where a gunfight erupted shortly after midnight, police said.

It was one of the heaviest casualty tolls suffered this year by the insurgents, who have fought a decades-long war against the Indian state from jungle hideouts across eastern and central India.

Police acted on a tip-off that around 30 rebels had gathered close to the border with Andhra Pradesh state, Odisha police chief K.B. Singh said.

He said there was no immediate information on police casualties. Officers had seized automatic rifles and ammunition from the scene.

"It was a joint operation by Andhra and Odisha police conducted last night... The operation is still going on," Singh said.

Maoist rebels accuse the Indian state of plundering the mineral-rich and underdeveloped east and central regions of the country at the expense of the poor and landless, among whom they retain some support.

While the level of violence has fallen in recent years, and the Maoists have lost hundreds of fighters to desertions and battles with security forces, the rebels remain capable of staging regular hit-and-run attacks across several states.

In July, Maoists killed at least 10 members of an elite Indian police force in the south of Bihar state.

According to the South Asia Terrorism Portal, 337 people have been killed in left-wing extremist violence so far this year, more than half of them alleged insurgents.